The present invention generally relates to Superconductive Quantum Interference Devices (SQUID), and in particular to SQUIDs for digital logic circuits.
With the steady demand for faster and more efficient communication and processing systems, many manufacturers are utilizing SQUIDs as digital circuit components in processing and communication systems. Generally, a SQUID is a two-terminal superconductive junction device capable of carrying a critical current through the junction. Critical current is defined as the maximum amount of current which the SQUID can carry with no voltage. The amount of current flowing through the SQUID can be modulated by a magnetic field threading the SQUID. The more intense the magnetic field, the lower the amount of critical current that can flow through the SQUID. Typically the magnetic field is generated by an inductor such as a control line carrying a control current. The intensity of the magnetic field can be controlled by the magnitude of the control current flowing through the control line.
Existing SQUIDs used in digital circuits have several important disadvantages however. Because critical current flow in a SQUID depends on the strength of the magnetic field applied to the SQUID, the sensitivity of the SQUID to the magnetic field, .beta..sub.L, is of paramount importance. A low .beta..sub.L generally indicates a more sensitive SQUID which is more responsive to variations in the magnetic field. For example, for a SQUID with a .beta..sub.L of 1 or less, a critical current of about 1 milliamp flowing through the SQUID can be modulated to about 0.1 milliamp by changing the intensity of the magnetic field applied to the SQUID. By contrast, for a SQUID with a .beta..sub.L of about 100, the critical current can only be reduced from 1 milliamp to 0.9 milliamp, regardless of the amount of change in the magnetic field applied to the SQUID. Therefore, a SQUID with a high .beta..sub.L is disadvantageous because of the limited critical current modulation possible.
Another disadvantage of existing SQUIDs is lack of uniformity of the SQUID step-edge junction characteristics, resulting in substantial variation in the amount of current flowing through the SQUID at different points across the SQUID junction. This is because in a step edge junction, the edges of the junction are unprotected. The exposed edges suffer from ion-damage during milling patterning processes, and are susceptible to contamination and damage from further processing by exposure to solvent and etchant chemicals. As such, the critical current density at the edges of the junction is nonuniform as compared to the rest of the junction. Typically, the best result for junction critical current uniformity for a 2 micron step-edge SQUID is about 20-30% 1 sigma.
Yet another disadvantage of existing SQUIDs is lack of linearity and uniformity of the SQUID critical current subjected to analog control signals via the SQUID control line. This makes existing SQUIDs particularly unsuitable for use in high-resolution analog-to-digital converters (ADC), for example. The quantizer in an ADC calibrates the input analog signal into discrete digital signal levels. In high-resolution ADCs, the input analog signal can be high in magnitude. For example, for a 10-bit ADC, if the equivalent least significant bit analog current is 10 microamps, then the most significant bit analog current is 10 milliamps. The lack of uniformity in linearity and critical current in existing SQUIDs substantially reduces the operation margin of ADC quantizers, making them unsuitable for proper handling of large current variations in input analog signals exemplared above.
There is therefore a need for a SQUID with a low .beta..sub.L for critical current modulation. There is also a need for a SQUID with uniform current density flowing across the SQUID junctions. There is also a need for a SQUID with linear and uniform critical current flow when subjected to analog control signals via the SQUID control line.